1. Field of the Invention
This invention relates to hybrid type rock bits.
More particularly, this invention relates to an extended nozzle multi-cone rock bit with drag bit type diamond cutters positioned in the face of the extended nozzle legs.
2. Description of the Prior Art
Hybrid bits of the type that utilize drilling mud to remove cuttings from the borehole are known in the art.
U.S. Pat. No. 4,006,788, assigned to the same assignee as the present invention, describes a rock bit for recovering core samples as well as rock bit variations for drilling oil wells or the like. In each of the several embodiments described, diamond cutters are strategically mounted on the bit body for cutting rock by a shearing action. Each diamond cutter is in the form of a thin diamond disc bonded to a tungsten carbide stud that is inserted into the bit body. Means are also provided for limiting the depth of penetration of the diamond cutters into the rock formation being drilled. For example, rolling cone cutters with a plurality of tungsten carbide inserts protruding from the surface of the cones limit penetration of the diamond cutters. The protrusion of the carbide inserts is less than the length of the diamond cutting face.
The foregoing patent is disadvantaged in that the multiplicity of diamond cutters placed on the various rock bit embodiments are not positioned to remove the ridges between kerfs left by the rows of tungsten carbide inserts in the roller cones as the cones traverse the bottom of a borehole.
U.S. Pat. No. 3,385,385 describes a roller bit for a large diameter borehole. A plurality of frustoconical cutters are mounted on a bit body, each cutter comprising rows of circumferential spaced-apart tungsten carbide inserts that form kerfs as the cutter traverses the borehole bottom. The same roller cone or cutter defines an intermediate disc-like row to dislodge and breakup the ridges between the kerfs. An alternate cutter apparatus includes a plurality of spaced-apart circumferential inserts positioned between the kerf cutting inserts to remove or breakup the ridges between the kerf rows.
The patent further teaches positioning of inserts at an angle to the web in which the inserts are mounted; each cutter having at least two webs, the angled inserts cutting a wide kerf in the borehole bottom.
A disadvantage with this arrangement of tungsten carbide inserts, while they cut a wide kerf, is the limiting of bit penetration due to relatively large borehole bottom area covered by the inserts in the cones.
Yet another disadvantage of the prior art is the inclusion of cutter elements and formation breaking means in the same cutter cone. Should the cone ball up, no following means is provided to continue bit penetration despite the balled up cone.
The present invention provides a following drag bit leg with diamond inserts so positioned on the face of the drag bit to independently remove the ridges between the deep kerfs cut by the inserts on the adjacent cones.
In addition, with prior art tungsten carbide chisel insert type rock bits, the inserts are typically placed on the cones with their chisel crest or crown oriented radially with respect to the cone to take advantage of the gouging, scraping action typically associated with this type of offset bit. The present invention orients the inner rows of inserts in the cones with their chisel crown circumferentially oriented on the cones so that they cut a narrower, deeper kerf in the borehole bottom, thus allowing the diamond inserts in the drag bit portion of the hybrid bit to work effectively to remove the ridges between the rows of inserts. By orienting the tungsten carbide chisels circumferentially on the cone, the penetration of each insert is deeper, resulting in a faster penetrating rock bit.